Composite materials which comprise an organic resin and an inorganic filler have been known and used for a number of years. Adhesion stability between the filler and the resinous mixture has been recognized as a source of degradation and failure of these materials for nearly as long as composite materials have been known. Coupling agents are additives which promote adhesion between the filler and resin and improve the hydrolytic stability of the bond between the two. Coupling agents in general have been known and used commercially since the introduction of chromium based coupling agents in the 1950s. Since the introduction of these coupling agents, there has been a steady development of new coupling agents which provide better strength and/or hydrolytic stability with various resin/filler combinations. The actual coupling capability of state of the art coupling agents is quite satisfactory. However, in general the best coupling agents in regards to adhesion and hydrolytic stability are also the most expensive coupling agents. Therefore, any coupling agent composition which gives premium performance at lower costs must be considered an advance in the art.
Surprisingly, it has been found that the addition of maleic anhydride, in certain proportions, to diamino functional silane coupling agents improves the performance of these silanes as coupling agents. Thus, a more effective coupling agent is provided which has the added advantage of being comprised of a substantial proportion of a low cost component (the maleic anhydride).
Aminofunctional silanes are well known in the art as effective coupling agents for epoxide, phenolic, melamine, furane, isocyanate, and other thermosettable resins. The reaction products of organic acids with aminofunctional silanes is also known in the art. For instance, U.S. Pat. No. 3,249,461 granted to Grotenhuis May 3, 1966 teaches the reaction product of an aminofunctional silane coupling agent with an acid chloride or anhydride in one to one amino to acid ratios. All of Grotenhuis's products are 1 to 1 amino radical to carboxy radical products, and are taught to be effective primers or coupling agents for olefinic resins.
U.S. Pat. No. 3,558,741 issued to Holub, et al. Jan. 26, 1971 teaches curable imido substituted organopolysiloxane compositions. Holub's compositions are the reaction product of one mole part aminofunctional silane and one mole part of an unsaturated anhydride such as maleic anhydride. The reaction is run to completion to form imides which Holub teaches are curable compositions.
U.S. Pat. No. 3,773,607 issued to Marzocchi Nov. 20, 1973 teaches silyl amides as "binding agents" between glass fibers or fillers and elastomers. Marzocchi's "anchoring agents" are the reaction product of a carboxy functional silane and an organic amine or an amino functional silane, where the ratio of carboxy groups to amine groups is between 0.8 and 3. Marzocchi does not mention stabilizing the resulting coupling agent composition.